1. Field of the Invention
The present invention relates to an advantageous coil as a thin-type coil for use in an electric device, and more particularly to a coil in which either of a first end of wire and a second end of wire is wound from an inner circumferential side towards an outer circumferential side, whereas the other end of wire is drawn forth from the inner circumferential side to the outer circumferential side.
2. Description of the Prior Art
An α winding method is generally known as a method for winding a coil by which a first end of wire and a second end of wire are both wound around from the inner circumferential side towards the outer circumferential side. With this winding method, as indicated in Japanese Patent Application Laid-open No. 2002-170729, a winding shaft is set close to the center at both ends of the wires and the first end of the wire and the second end of the wire are wound in mutually opposite directions, whereby a coil can be formed in which both the first end of the wire and the second end of the wire are drawn forth to the outside.
A method is also known by which a winding start end side of a coil is fixed, the winding completion end side is wound from the inner circumferential side towards the outer circumferential side and the winding start end is drawn forth from the inner circumferential side towards the outer circumferential side. In this case, the winding start end is dragged over the flat surface of the wound coil, and the draw-forth line portion of the winding start end side becomes larger in size (thickness) in the height direction by the diameter of the winding wire at the winding start end. A known measure for resolving this problem is to provide a concavity on the flat surface of the wound coil along the draw-forth line of the winding start end on the flat surface of the wound coil (see Japanese Patent Application Laid-open No. 2006-049750).
However, the problem related to the coil described in Japanese Patent Application Laid-open No. 2002-170729 is that because the first end of the wire and the second end of the wire are wound in the mutually opposite directions and the winding wires are piled up in two layers at the crossing portions thereof, a portion with a height twice the diameter of the wires appears along the crossing portions. Further, when the coil is an air-core coil, problems are related to the coil strength because of the structure in which the winding wires are piled up in two layers.
Further, with the α winding method described in Japanese Patent Application Laid-open No. 2002-170729, where winding is wound in a state in which the length of the first end of the wire portion is substantially equal to that of the second end of the wire portion, when the coil with a large number of turns (a large length of winding wires) is wound, fliers 11a, 11b, which are on the side where rotation is caused, have to be large and the size of the winding apparatus itself becomes large.
In the case of the coil described in Japanese Patent Application Laid-open No. 2006-049750, since the flat surface of the wound coil is concaved along the draw-forth line of the winding start end side from the inner circumferential side towards the outer circumferential side, stresses are applied to the flat surface of the coil and the coating on the winding wire can be damaged or the winding wire can be broken. Further, due to concavity on the flat surface of the wound coil, there may be a problem that the winding wire protrudes at other portions.